fix(gateway): flush throttled delta before emitChatFinal (#24856)

* fix(gateway): flush throttled delta before emitChatFinal

The 150ms throttle in emitChatDelta can suppress the last text chunk
before emitChatFinal fires, causing streaming clients (e.g. ACP) to
receive truncated responses. The final event carries the complete text,
but clients that build responses incrementally from deltas miss the
tail end.

Flush one last unthrottled delta with the complete buffered text
immediately before sending the final event. This ensures all streaming
consumers have the full response without needing to reconcile deltas
against the final payload.

* fix(gateway): avoid duplicate delta flush when buffer unchanged

Track the text length at the time of the last broadcast. The flush in
emitChatFinal now only sends a delta if the buffer has grown since the
last broadcast, preventing duplicate sends when the final delta passed
the 150ms throttle and was already broadcast.

* fix(gateway): honor heartbeat suppression in final delta flush

* test(gateway): add final delta flush and dedupe coverage

* fix(gateway): skip final flush for silent lead fragments

* docs(changelog): note gateway final-delta flush fix credits

---------

Co-authored-by: Jonathan Taylor <visionik@pobox.com>
Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

src/commands/agent.ts

@@ -873,6 +873,10 @@ async function agentCommandInternal(
       fallbackProvider = fallbackResult.provider;
       fallbackModel = fallbackResult.model;
       if (!lifecycleEnded) {
+        const stopReason = result.meta.stopReason;
+        if (stopReason && stopReason !== "end_turn") {
+          console.error(`[agent] run ${runId} ended with stopReason=${stopReason}`);
+        }
         emitAgentEvent({
           runId,
           stream: "lifecycle",
@@ -881,6 +885,7 @@ async function agentCommandInternal(
             startedAt,
             endedAt: Date.now(),
             aborted: result.meta.aborted ?? false,
+            stopReason,
           },
         });
       }